Method for preparing metal phosphide nanocrystal from phosphite compound and method for passivating nanocrystal core with the same

ABSTRACT

Disclosed herein is a method for the preparation of metal phosphide nanocrystals using a phosphite compound as a phosphorous precursor. More specifically, disclosed herein is a method for preparing metal phosphide nanocrystals by reacting a metal precursor with a phosphite compound in a solvent. A method is also provided for passivating a metal phosphide layer on the surface of a nanocrystal core by reacting a metal precursor with a phosphite compound in a solvent. The metal phosphide nanocrystals have uniform particle sizes and various shapes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.11/970,972 filed Jan. 8, 2008, which claims priority to Korean PatentApplication No. 2007-37385 filed on Apr. 17, 2007 in the KoreanIntellectual Property Office, the disclosure of each of which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a method for preparing metalphosphide nanocrystals from a phosphite compound, and a method forpassivating a nanocrystal core with the same. More specifically, thepresent invention relates to a method for producing metal phosphidenanocrystals and a method for passivating a nanocrystal core, which canbe conducted under more controlled conditions via the introduction of aphosphite compound as a phosphorous precursor.

2. Description of the Related Art

Nanocrystals exhibit electrical, magnetic, optical, chemical andmechanical properties that are distinguished from those of bulkysubstances. Since the properties of nanocrystals are readilycontrollable depending upon the size and composition of thenanocrystals, there has been extensive interest in nanocrystals. Basedon their luminescent and electrical properties alone, nanocrystals canbe utilized in a variety of applications including light-emitting andlight-receiving devices, solar cells, sensors and lasers.

Vapor deposition processes, including metal organic chemical vapordeposition (MOCVD) and molecular beam epitaxy (MBE), have been used toprepare nanocrystals. In recent years, there have been a number ofstudies conducted to investigate the preparation of semiconductornanocrystals using a wet chemistry method wherein a precursor materialis added to a coordinating organic solvent in order to grow thenanocrystals to a desired size.

As the crystals are grown, the organic solvent is naturally coordinatedto the surface of the nanocrystals, thereby acting as a dispersant toadjust the crystals to a nanometer-scale level. The wet chemistry methodhas advantages in that nanocrystals of a variety of sizes can beuniformly prepared by appropriately controlling the concentration ofprecursor materials used, the type of organic solvents used, as well asthe preparation temperature and time. In particular, since Group II-VIcompound semiconductor nanocrystals can emit light in the visible rangeand, since they can be prepared in a simple manner as compared to GroupIII-V compound semiconductor nanocrystals, they are actively understudy.

Since Group III-V compound semiconductor nanocrystals have a covalentbond, they exhibit superior stability and relatively low toxicity ascompared to Group II-VI compound semiconductor nanocrystals that have anionic bond. However, Group III-V compound semiconductor nanocrystalshave disadvantages associated with their synthesis such as, for example,a long synthesis time and a limited number of potential precursors.

In particular, synthesis methods for metal phosphide nanocrystals (e.g.,indium phosphide (InP) or gallium phosphide (GaP)) are being activelystudied. According to prior art wet chemistry methods, trimethylsilylphosphine (“(TMS)₃P”) is the only phosphorus precursor used. However,(TMS)₃P has a very high reactivity, thus making it difficult to controla reaction under predetermined conditions.

Accordingly, there is an increasing demand for a wet chemistry synthesismethod that enables the preparation of metal phosphide nanocrystalsunder more controlled conditions.

SUMMARY OF THE INVENTION

In one embodiment, a method is provided for preparing metal phosphidenanocrystals by wet chemical synthesis, wherein the method comprisesreacting a metal precursor with a phosphite compound in a solvent.

In another embodiment, the method for preparing the metal phosphidenanocrystals comprises adjusting a temperature of a metal precursorsolution comprising a metal precursor, a dispersant, and a solvent to apredetermined temperature; preparing a phosphate compound solution;feeding the phosphate compound solution to the metal precursor solutionand reacting the mixture, to grow metal phosphide nanocrystals; andseparating the metal phosphide nanocrystals from the mixture.

In yet another embodiment, a method is provided for passivating ananocrystal core wherein the method comprises adding a metal precursorand a phosphite compound to a solution comprising a nanocrystal core andreacting the mixture to passivate a metal phosphide layer on the surfaceof the nanocrystal core.

In one embodiment, the method for passivating a nanocrystal corecomprises preparing a nanocrystal core solution comprising a nanocrystalcore, a metal precursor, and a dispersant; preparing a phosphitecompound solution comprising a phosphite compound in a solvent; feedingthe phosphate compound solution to the nanocrystal core solution andreacting the mixture to prepare metal phosphide nanocrystals; andseparating the metal phosphide nanocrystals from the mixture.

In another embodiment, metal phosphide nanocrystals are provided asprepared by the foregoing methods.

In accordance with one aspect, there is provided an inorganic-organichybrid electroluminescence device comprising a plurality of organic andinorganic layers interposed between a pair of electrodes wherein theorganic layer comprises a luminescent layer comprising the metalphosphide nanocrystals according to the present invention.

In accordance with another aspect, the method provides a nanocrystalwherein the shape of the metal phosphide nanocrystals can be readilycontrolled by introduction of a phosphite compound as a phosphorousprecursor.

In accordance with another aspect, the method provides the use of a widevariety of for the formation of the nanocrystals of solvents, since thephosphite compound is highly soluble in numerous solvents.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is an exemplary schematic diagram illustrating a method forpreparing metal phosphide nanocrystals;

FIG. 2 is a UV absorption spectrum of the metal phosphide nanocrystalssynthesized in Examples 1 and 2;

FIGS. 3A and 3B are transmission electron micrographs (TEM) of metalphosphide nanocrystals synthesized as described in Examples 1 and 2respectively;

FIG. 4A is a UV absorption spectrum for the nanocrystals obtained beforeand after the formation of a metal phosphide passivation layersynthesized as described in Example 3;

FIG. 4B is a photoluminescence spectrum for the nanocrystals after theformation of a metal phosphide passivation layer synthesized asdescribed in Example 3;

FIG. 5 is a transmission electron micrograph (TEM) of the nanocrystalsafter the formation a metal phosphide passivation layer as described inExample 3; and

FIG. 6 is a UV absorption spectrum of the metal phosphide nanocrystalssynthesized as described in Example 2, and in Comparative Examples 1 and2.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in more detail withreference to the accompanying drawings.

It will be understood that when an element or layer is referred to asbeing “on,” “interposed,” “disposed,” or “between” another element orlayer, it can be directly on, interposed, disposed, or between the otherelement or layer or intervening elements or layers may be present.

It will be understood that, although the terms first, second, third, andthe like may be used herein to describe various elements, components,regions, layers and/or sections, these elements, components, regions,layers and/or sections should not be limited by these terms. These termsare only used to distinguish one element, component, region, layer orsection from another element, component, region, layer or section. Thus,first element, component, region, layer or section discussed below couldbe termed second element, component, region, layer or section withoutdeparting from the teachings of the present invention.

As used herein, the singular forms “a,” “an” and “the” are intended tocomprise the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

The method described herein utilizes a phosphite compound as a novelphosphorus precursor for the preparation of metal phosphidenanocrystals. Specifically, when a phosphite compound solution is mixedwith a solution comprising at least one metal precursor, the phosphitecompound, as the phosphorus precursor, reacts with the metal precursorin the mixed solution resulting in the formation of metal phosphidenanocrystals.

According to one embodiment, the metal phosphide nanocrystals have auniform particle size at the nanometer-scale level, selectively desiredcrystal structures, and various shapes, as a result of the use of thephosphite compound as a phosphorus precursor metal.

As mentioned previously, trimethylsilyl phosphine ((TMS)₃P), has beengenerally used as a phosphorus precursor in wet chemistry methods.(TMS)₃P contains electron-donating ligands, making the centralphosphorus atom highly nucleophilic. In addition, since (TMS)₃P containsP-Si bonds which are easily cleavable due to their weak bondingstrength, (TMS)₃P is a highly reactive phosphorus compound capable ofreadily forming metal phosphide bonds upon reaction with a metal. Forthis reason, the reaction of (TMS)₃P with a metal precursor cannot beappropriately controlled at a high temperature. Conversely, the use of aphosphite compound as a phosphorus precursor, as disclosed herein,results in the formation of a stable P—O bond, and as such, the reactioncan be carried out under controlled conditions. Specifically, in thecase where nucleophilic (TMS)₃P is used as a phosphorus precursor, thereaction time can be controlled within about 5 seconds (sec) to about 10minutes (min) while the reaction temperature is maintained at about 300°Celsius (C). On the other hand, in a case where a phosphite compound isused as a phosphorus precursor, the reaction time can be adjusted toabout 20 sec to about 60 min while the reaction temperature ismaintained at about 300° C.

Accordingly, the method described herein has an advantage in that thereaction conditions can be controlled over a wider-ranging period oftime. In addition, because P—Si bonds are cleaved, even in the absenceof the metal precursor, the reaction of (TMS)₃P with the metal precursormust be proceed under conditions wherein the amount of (TMS)₃P issmaller than that of the metal precursor, in order to minimize theformation of a phosphorous aggregate at a high temperature. However, thephosphite compound is stable, and can thus be utilized independent ofthe concentration of the metal precursor. Accordingly, the use of thephosphite compound as a phosphorous precursor enables the preparation ofnanocrystals in a desired shape and size, under more varied conditions.

Since the phosphite compound remains on the surface of the grownnanocrystals, the surface properties of the nanocrystals can also becontrolled depending on the type of functional groups that are includedin the phosphite compound, without any additional surface displacement.

That is, it is estimated that the phosphite compound acts as aphosphorous precursor during the preparation of metal phosphidenanocrystals, and furthermore, any unreacted phosphite compound acts asa kind of dispersant surrounding the surface of the nanocrystals.Accordingly, the use of at least one phosphite compound having varioustypes of functional groups, enables the preparation of metal phosphidenanocrystals whose surface properties (e.g. polarity and bindingproperties) are controlled without resulting in additional surfacedisplacement.

The phosphite compound described herein is miscible with a variety ofsolvents, depending upon the functional groups that are included in thephosphite compound. Accordingly, the method described herein enables theselection of a suitable solvent from a variety of types of solvents. Inparticular, the method has a distinct advantage in that a nontoxicsolvent can be used.

In addition, in the case where metal phosphide nanocrystals are grown onthe surface of other kinds of nanocrystals, defects present on thesurface can be protected, and thus the optical and electrical propertiesof the nanocrystals can be controlled.

In one embodiment, a method is provided for preparing a metal phosphidenanocrystal by chemical wet synthesis, wherein the method comprisesreacting a metal precursor with a phosphite compound in a solvent.

The method for preparing a metal phosphide nanocrystal comprises:adjusting the temperature of a metal precursor solution comprising ametal precursor and a dispersant to a predetermined temperature;preparing a reaction solution of a phosphite compound; feeding thephosphite compound solution to the metal precursor solution and,reacting the mixture, to prepare metal phosphide nanocrystals; andseparating the metal phosphide nanocrystals from the mixture.Accordingly, the method can be variously modified and altered byprocesses known in the art.

Examples of metal precursors include, organometallic compounds having anelement selected from Zn, Cd, Hg, Pb, Sn, Ge, Ga, In, Tl, Sc, Ti, V, Cr,Mn, Fe, Co, Ni, Cu, Y, Zr, Nb, Mo, Tc, Pd, Ag, Pt, Au, or salts thereof,or a combination comprising at least one of the foregoing elements.Specifically, the use of organometallic compounds having a Group IIImetal element.

Examples of specific organometallic compounds having a Group III metalelement include, gallium acetylacetonate, gallium chloride, galliumfluoride, gallium oxide, gallium nitrate, gallium sulfate, indiumacetate, indium acetylacetonate, indium chloride, indium oxide, indiumnitrate, indium sulfate, thallium acetate, thallium acetylacetonate,thallium chloride, thallium oxide, thallium ethoxide, thallium nitrate,thallium sulfate, thallium carbonate, or alloys, or a combinationcomprising at least one of the foregoing organometallic compounds.

The phosphite compound is represented by Formula 1 below:

P(OR)₃   [Formula 1]

In Formula 1, R is selected from a substituted or an unsubstitutedC₁-C₂₀ alkyl, aryl, ether, ethylene, oxide and propylene oxide.

When R is a substituted alkyl, the alkyl of the phosphite compoundcomprises at least one functional group at an intermediate or terminalposition. The functional group may be selected from unsaturated,carboxyl, amide, phenyl, amine, acryl, silane, phosphine, phosphinicacid, cyano and thiol groups. The phosphite compound may be used aloneor as a combination comprising at least one of the foregoing functionalgroups.

The reaction solvent used herein may be selected from organic solventsgenerally used in the art. Examples of reaction solvents include primaryalkyl amines, secondary alkyl amines, heterocyclic compounds containingat least one nitrogen or sulfur atom, alkanes, alkenes, alkynes,trioctylphosphine, and trioctylphosphine oxide. Other examples ofreaction solvents include polar primary alcohols, secondary alcohols,tertiary alcohols, ketones esters. Furthermore, aqueous solutions, andcombinations of both aqueous solutions and organic solvents, may be usedas reaction solvents.

Examples of suitable dispersants that can be used in the method includecarboxyl acids, for example, oleic acid, stearic acid and palmitic acid;organic phosphorus acids, for example, hexyl phosphonic acid, n-octylphosphonic acid, tetradecyl phosphonic acid and octadecyl phosphonicacid; and amines, for example, n-octyl amine, hexadecyl amine. Since thephosphite compound acts as both a phosphorus precursor and as adispersant, the method of the present invention can avoid the use of adispersant.

The phosphite compound is preferably diluted to about 0.001 Molar (M) toabout 1M in a solvent. Examples of solvents that can be used to dilutethe phosphite compound include solvents having a low boiling point, suchas dimethyl chloride, toluene, hexane, heptane, octane, pyridine andbutanol, in addition to the reaction solvents.

The phosphite compound is stable, thus advantageously allowing theuniform preparation of a nanocrystal at a high temperature. The reactiontemperature is about 80° C. to about 400° C., specifically about 150° C.to about 350° C., and more specifically about 200° C. to about 350° C.The reaction time may vary depending on the type of metal used and thephosphite compound, and is preferably about one second to about one day.

In another embodiment, a method is provided for passivating ananocrystal core with a metal phosphide layer, wherein the methodcomprises adding a metal precursor and a phosphite compound to asolution comprising a nanocrystal core, and reacting the mixture.

According to one embodiment, the passivation of the nanocrystal core canbe carried out with the same constituent components as in thenanocrystal preparation. The passivation method may comprise preparing ananocrystal core solution comprising a nanocrystal core, a metalprecursor, and a dispersant; preparing a phosphite compound solutioncomprising a phosphite compound; adding the phosphite compound solutionprepared to the nanocrystal core solution and reacting the mixture toprepare metal phosphide nanocrystals; and, separating the metalphosphide nanocrystal from the mixture. Accordingly, the method of thepresent invention can be variously modified and altered by processesknown in the art.

Since the phosphite compound is highly reactive with the surface of thecore nanocrystals, the addition of the phosphite compound to thenanocrystal core solution permits the phosphite compound to selectivelysurround the surface of the nanocrystal core. If a metal precursor isincluded in the solution, the metal precursor starts to react with thephosphite compound to grow metal phosphide nanocrystals on the coresurface. The grown metal phosphide layer removes any defects present onthe surface of the nanocrystals, and changes the surface characteristicsof the nanocrystals.

Nanocrystals used as the core herein are not limited to compoundsemiconductor materials alone, and include metal oxide nanocrystals andmetal nanocrystals on which a phosphite compound can be adsorbed.Examples of specific core nanocrystals include semiconductor compoundnanocrystals selected from CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe,HgTe, GaN, GaP, GaAs, InP or InAs; metal oxide nanocrystals selectedfrom TiO₂, SiO₂, CdO, Fe₂O₃, CuO, AgO or ZrO; and metal nanocrystalsselected from Pt, Pd, Ru, Rh, Ir, Au, Ag, Fe, Co, Ni, Si or Ge; or,combinations comprising at least one of the foregoing semiconductor,metal oxide or metal nanocrystals.

The nanocrystal core may have may have various shapes selected fromspheres, rods, tripods, tetrapods, cubes, boxes, stars, or acombinations comprising at least one of the foregoing shapes.

According to another embodiment, the present invention is directed tometal phosphide nanocrystals prepared by the method.

The metal phosphide nanocrystal, or the metal phosphide layer formed onthe nanocrystal core, comprise a metal phosphide selected from AlP, InP,GaP, Zn₃P₂, Cd₃P₂, MnP, FeP, Fe₂P, Co₂P or Ni₂P, or a combinationcomprising at least one of the foregoing metal phosphides. If two ormore metal phosphides are present as nanocrystals, they may be partiallylocalized or may be present in a mixture or an alloy form.

The metal phosphide nanocrystals may have various shapes selected fromspheres, rods, tripods, tetrapods, cubes, boxes, stars, or a combinationcomprising at least one of the foregoing shapes.

The metal phosphide nanocrystal, and the nanocrystal passivated by themetal phosphide layer, can be utilized in a variety of applicationsincluding, for example, displays, sensors, and electronic devices. Inparticular, these nanocrystals are useful for the production of alight-emitting layer for an organic/inorganic hybrid electroluminescentdevice. The introduction of the semiconductor nanocrystals into thelight-emitting layer, can be carried out using common processes known inthe art, including vapor deposition, sputtering, printing, coating andelectron beam processes. The light-emitting layer preferably has athickness of about 50 to about 100 nanometers (nm). In addition to thelight-emitting layer, organic layers present in an inorganic-organichybrid electroluminescence device, include an electron transport layerand a hole transport layer, both of which are interposed between a pairof electrodes and are composed of organic compounds.

The electroluminescent device described above may comprise layersdisposed in a variety of structures known in the art. Examples ofstructures include, anode/light-emitting layer/cathode, anode/bufferlayer/light-emitting layer/cathode, anode/hole transportlayer/light-emitting layer/cathode, anode/buffer layer/hole transportlayer/light-emitting layer/cathode, anode/buffer layer/hole transportlayer/light-emitting layer/electron transport layer/cathode, oranode/buffer layer/hole transport layer/light-emitting layer/holeblocking layer/cathode, or a combination comprising at least one of theforegoing layer structures.

Materials for the buffer layer comprise compounds commonly known in theart. Examples of buffer layer materials include, copper phthalocyanine,polythiophene, polyaniline, polyacetylene, polypyrrole, polyphenylenevinylene or derivatives thereof, or a combination comprising at leastone of the foregoing buffer layer materials.

Materials for the hole transport layer comprise compounds commonly knownin the art. Specifically, polytriphenylamine can be used, but thepresent invention is not limited thereto.

Materials for the electron transport layer comprise compounds commonlyknown in the art. Specifically, polyoxadiazole can be used, but thepresent invention is not limited thereto.

Materials for the hole blocking layer comprise compounds commonly knownin the art. Examples of materials for the hole blocking layer includeLiF, BaF₂, MgF₂, and the like.

The organic/inorganic hybrid electroluminescent device described hereindoes not require particular fabrication apparatuses and methods, and canbe fabricated in accordance with prior art methods for organicelectroluminescence devices, using common materials.

Hereinafter, the present invention will be explained in more detail withreference to the following examples. However, these examples are givenfor the purpose of illustration and are not to be construed as limitingthe scope of the invention.

EXAMPLES Example 1 Synthesis of InP Nanocrystals from aTriethylphosphite Compound at about 300° C.

Ten grams (g) of octadecene (“ODE”), 0.2 g of oleic acid, and 0.2 mmolof indium acetate were put in a 125 ml flask equipped with a refluxcondenser. The mixture was reacted with stirring until the reactiontemperature reached about 300° C.

Separately, about 70 microliters (μL) of triethylphosphite was dissolvedin 1 mL of ODE to prepare a solution. The solution was rapidly added tothe reaction mixture. The mixture was reacted with stirring for aboutone hour.

After the completion of the reaction, the reaction temperature wascooled to room temperature as quickly as possible. Ethanol as anon-solvent was added to the cooled reaction mixture, and the resultingmixture was centrifuged. The suspension was decanted away to obtain aprecipitate. The precipitate was dispersed in 5 mL of toluene to yield asolution of InP nanocrystals. FIG. 2 shows the UV absorption spectrumfor the InP nanocrystals. As shown in FIG. 2, a clearly dissolved peakdemonstrates that the InP nanocrystals were produced with a constantsize and shape. FIG. 3A is a transmission electron micrograph (TEM) ofthe InP nanocrystals.

Example 2 Synthesis of InP Nanocrystals from a TriethylphosphiteCompound at about 320° C.

Ten g of ODE, 0.2 g of oleic acid and 0.2 mmol of indium acetate wereput in a 125 ml flask equipped with a reflux condenser. The mixture wasreacted with stirring until the reaction temperature reached about 320°C.

Separately, about 70 pL of triethylphosphite was dissolved in 1 mL ofODE to prepare a solution. The solution was rapidly added to thereaction mixture. The mixture was reacted with stirring for about onehour.

After completion of the reaction, the reaction temperature was cooled toroom temperature as quickly as possible. Ethanol as a non-solvent wasadded to the cooled reaction mixture, and the resulting mixture was thencentrifuged. The suspension was decanted away to obtain a precipitate.The precipitate was dispersed in 5 mL of toluene to yield a solution ofInP nanocrystals. FIG. 2 shows the UV absorption spectrum for the InPnanocrystals. As shown in shown in FIG. 2, a clearly dissolved peakdemonstrates that the InP nanocrystals have superior crystallinity ascompared to the sample produced in Example 1. FIG. 3B is a transmissionelectron micrograph (TEM) of the InP nanocrystals.

Example 3 Synthesis of CdSe/CdS Nanocrystal and Formation of InPPassivation Layer

Sixteen g of trioctyl amine (“TOA”), 2.0 g of oleic acid, and 1.6 mmolof cadmium oxide were put in a 125 ml flask equipped with a refluxcondenser. The mixture was reacted with stirring until the reactiontemperature was adjusted to about 300° C.

Separately, a selenium (Se) powder was dissolved in trioctyl phosphine(“TOP”) to prepare a Se-TOP complex solution (Se concentration: ca. 0.1M). The Se-TOP complex solution was rapidly added to the stirringreaction mixture and further reacted for about 2 min. To the reactionmixture a solution of octane thiol (0.06 g) in TOA (2 mL) was slowlyadded. The reaction mixture was allowed to stand at the same temperature(about 300° C.) for about 30 minutes.

After completion of the reaction, the reaction temperature was cooled toroom temperature as quickly as possible. Ethanol as a non-solvent wasadded to the cooled reaction mixture, and the resulting mixture was thencentrifuged. The suspension was decanted away to obtain a precipitate.The precipitate was dispersed in 5 mL of toluene to prepare a solutionof CdSe/CdS nanocrystals.

Ten g of ODE, 0.04 g of oleic acid and 0.04 mmol of indium acetate wereput in a 125 ml flask equipped with a reflux condenser. The mixture wasreacted with stirring until the reaction temperature was adjusted toabout 300° C.

To the reaction mixture the CdSe/CdS nanocrystal solution, and asolution of triethyl phosphite (about 70 pL) in ODE (1 mL), weresequentially added. The mixture was reacted for about one hour.

After completion of the reaction, the reaction temperature was cooled toroom temperature as quickly as possible. Ethanol as a non-solvent wasadded to the cooled reaction mixture, and the resulting mixture was thencentrifuged. The suspension was decanted away to obtain a precipitate.The precipitate was dispersed in 5 mL of toluene. FIG. 4A shows the UVabsorption spectrum for the CdSe/CdS nanocrystals before and after theformation of an InP passivation layer. It can be seen in FIG. 4A thatthe UV absorption spectrum of the CdSe/CdS nanocrystal core wasmaintained following the formation of the InP passivation layer. FIG. 4Bshows the photoluminescence spectrum of the nanocrystals following theformation of the InP passivation layer. A luminescence peak was observedat about 592 nm. FIG. 5 is a transmission electron micrograph (TEM) ofthe CdSe/CdS/InP nanocrystals in which the InP passivation layer isformed on the CdSe/CdS nanocrystals.

Comparative Example 1 Synthesis of InP Nanocrystal fromTrioctylphosphine Compound

Ten g of ODE, 0.2 g of oleic acid and 0.2 mmol of indium acetate wereput in a 125 ml flask equipped with a reflux condenser. The mixture wasreacted with stirring until the reaction temperature was adjusted toabout 320° C.

About 0.2 g of trioctylphosphine was dissolved in 1 mL of ODE to preparea solution. The solution was rapidly added to the reaction mixture. Themixture was reacted with stirring for about 20 min.

After completion of the reaction, the reaction temperature was cooled toroom temperature as quickly as possible. Ethanol as a non-solvent wasadded to the cooled reaction mixture, and the resulting mixture was thencentrifuged. The suspension was decanted away to obtain a precipitate.The precipitate was dispersed in 5 mL of toluene. The dispersion wasanalyzed by UV absorption spectroscopy, and the results are shown inFIG. 6. The fact that there is no peak corresponding to InP demonstratesthat the InP nanocrystals were not formed.

Comparative Example 2 Synthesis of InP Nanocrystal fromTributylphosphine Compound

Ten g of ODE, 0.2 g of oleic acid and 0.2 mmol of indium acetate wereput in a 125 ml flask equipped with a reflux condenser. The mixture wasreacted with stirring until the reaction temperature was adjusted toabout 320° C.

About 0.1 g of trioctylphosphine was dissolved in 1 mL of ODE to preparea solution. The solution was rapidly added to the reaction mixture. Themixture was reacted with stirring for about 20 min.

After completion of the reaction, the reaction temperature was cooled toroom temperature as quickly as possible. Ethanol as a non-solvent wasadded to the cooled reaction mixture, and the resulting mixture was thencentrifuged. The suspension was decanted away to obtain a precipitate.The precipitate was dispersed in 5 mL of toluene. The dispersion wasanalyzed by UV absorption spectroscopy and the results are shown in FIG.6. The fact that there is no peak corresponding to InP was not observedindicates the InP nanocrystals were not formed. Further, this result wasconfirmed in that, when phosphine-based compounds having a highly stableP—C bond were used, no InP was formed.

As is apparent from the foregoing examples, the exemplary metalphosphide nanocrystals can be prepared under more controlled conditionsvia the use of a phosphite compound as a phosphorous precursor. As aresult, the metal phosphide nanocrystals thus prepared have selectivelydesired crystal structures and a variety of shapes. Furthermore, thesurface properties of the metal phosphide nanocrystals can be controlleddepending upon the functional groups that are included in the phosphitecompound. In addition, the phosphite compound is miscible in a varietyof solvents, thus allowing for its use under various reactionconditions.

Through the formation of a metal phosphide layer made of a phosphidecompound on the surface of the nanocrystals, the optical and electricalproperties of the nanocrystals can be controlled and thus, significantlyimproved luminescence efficiency can be realized.

Reference throughout the specification to “one embodiment”, “anotherembodiment”, “an embodiment”, and so forth, means that a particularelement (e.g. feature, structure, and/or characteristic) described inconnection with the embodiment is included in at least one embodimentdescribed herein, and may or may not be present in other embodiments. Inaddition, it is to be understood that the described elements may becombined in any suitable manner in the various embodiments.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. Metal phosphide nanocrystals prepared by: adjusting a temperature ofa metal precursor solution to a predetermined temperature, the metalprecursor solution comprising a metal precursor, a dispersant and asolvent; preparing a phosphite compound solution, wherein the phosphitecompound is represented by Formula 1 below:P(OR)₃   (Formula 1) wherein R is selected from the group consisting ofa substituted or an unsubstituted C₁-C₂₀ alkyl, aryl, ether, ethylene,oxide, and propylene oxide; feeding the phosphite compound solution tothe metal precursor solution to form a mixture; reacting the mixture togrow metal phosphide nanocrystals; and separating the metal phosphidenanocrystals from the mixture.
 2. The metal phosphide nanocrystals ofclaim 1, wherein the metal precursor is an organometallic compoundhaving an element selected from the group consisting of Zn, Cd, Hg, Pb,Sn, Ge, Ga, In, Tl, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Nb, Mo,Tc, Pd, Ag, Pt and Au, a salt of Zn, Cd, Hg, Pb, Sn, Ge, Ga, In, Tl, Sc,Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Nb, Mo, Tc, Pd, Ag, Pt, and Au,and a combination comprising at least one of the foregoing elements orsalts.
 3. The metal phosphide nanocrystals of 1, wherein the metalprecursor is selected from the group consisting of galliumacetylacetonate, gallium chloride, gallium fluoride, gallium oxide,gallium nitrate, gallium sulfate, indium acetate, indiumacetylacetonate, indium chloride, indium oxide, indium nitrate, indiumsulfate, thallium acetate, thallium acetylacetonate, thallium chloride,thallium oxide, thallium ethoxide, thallium nitrate, thallium sulfate,thallium carbonate, alloys thereof, and combinations comprising at leastone of the foregoing organometallic compounds.
 4. The metal phosphidenanocrystals of claim 1, wherein R of Formula 1 is a substituted alkylwith a functional group selected from the group consisting ofunsaturated, carboxyl, amide, phenyl, amine, acryl, silane, phosphine,phosphinic acid, cyano, and thiol groups.
 5. The metal phosphidenanocrystals of claim 1, wherein the metal phosphide nanocrystalscomprise a metal phosphide compound selected from the group consistingof AlP, InP, GaP, Zn₃P₂, Cd₃P₂, MnP, FeP, Fe₂P, Co₂P Ni₂P, and acombination comprising at least one of the foregoing metal phosphidecompounds.
 6. The metal phosphide nanocrystals of claim 1, wherein themetal phosphide nanocrystals have a shape selected from the groupconsisting of a sphere, rod, tripod, tetrapod, cube, box, star, or acombination comprising at least one of the foregoing shapes.
 7. Themetal phosphide nanocrystals of claim 1, wherein the solventis selectedfrom the group consisting of primary alkyl amines, secondary alkylamines, heterocyclic compounds containing at least one nitrogen orsulfur atom, alkanes, alkenes, alkynes, trioctylphosphine,trioctylphosphine oxide, polar primary alcohols, secondary alcohols,tertiary alcohols, ketones and esters, aqueous solutions, and acombination comprising at least one of the foregoing solvents.
 8. Themetal phosphide nanocrystals of claim 1, wherein the reacting isconducted at a temperature of about 200° C. to about 350° C.